(1) Field of the Invention
This invention relates to dichroic coloring agents having a blue to blue violet hue for use in liquid crystals.
(2) Description of the Prior Art
Recently, liquid crystal display elements have been widely used for a variety of variable displays from the energy saving and miniaturization viewpoints. Most of the liquid crystal display elements now being used utilize the electro-optic effect of twisted nematic type liquid crystals. A combination of two polarizing plates is required in this type of display systems and actually causes many limitations in use. The so-called guest-host system has been examined as an alternative liquid crystal display system. The guest-host system utilizes the electro-optic effect of a colored liquid crystal composition obtained by dissolving a dichroic coloring agent in a nematic liquid crystal. The guest-host system has already been utilized in part as display elements for clocks, electric and electronic appliances, industrial meters and other uses.
The principle of the guest-host type liquid crystal display system is to arrange guest molecules of the dichroic coloring agent in oriented states depending on the configuration of the host molecules of the liquid crystal. The molecules of liquid crystals change direction of their orientation from the OFF state to the ON stage usually by applying the external stimulus of an electric field. At the same time, the molecules of the dichroic coloring agent also change the direction of their orientation. Consequently, the degree of light absorption by the molecules of the coloring agent varies between the OFF and ON states and excites the display, which is the principle of the guest-host system.
The minimum requirements for the dichroic coloring agent to be used are to provide such essential conditions as:
(a) having sufficient tinting strength in small amounts, PA0 (b) exhibiting a sharp contrast, PA0 (c) having sufficient compatibility with the liquid crystal, and PA0 (d) being excellent in durability and stability and not degrading performance of the display unit after long use.
Various kinds of dichroic coloring agents have been proposed to meet the above conditions, and some of them have already been used for digital clocks, meters and other applications. However, coloring agents exhibiting a high dichroic ratio are poor in durability, whereas those having excellent durability are too weak in dichroism to exhibit a clear display in practice. Many of these coloring agents still have disadvantages to be overcome.